1. Trim ends off beans and cut into 2-inch lengths.
Cut zucchini in half, lengthwise, then into 11/2 inch pieces.
Core pepper, cut away veins and cut into 1/2 inch strips.
2. Bring enough water to boil to cover beans and zucchini when added. Add beans and simmer 5 minutes.
3. Add zucchini and pepper strips and cook 2 minutes. Drain well.
4. Run vegetables under cold water to chill well and drain.
5. Whisk mustard, vinegar, garlic, salt and pepper in a small bowl. Add oil, slowly, beating vigorously until emulsified.
6. Arrange vegetables and toss with lightly with dressing (you will not need all of it), add red onion and basil and toss again.